Patch problem - I don't have a previous version?

[GG 1]

Ok, I bought Civ 4 2-3 days ago from www.direct2drive.com , and I went to install the patch and according to it "You don't have a previous version of the game installed"... But.. Umm.. I do. Yeah, no idea what the problem may be except for that it was from that site? If I'm not going to be able to get patches, that is really, really, lame. Also, if someone can find out what the problem is is it even worth it to download the patch? I have heard that it's pretty buggy so I'm not too sure that I should

Thanks